The factors that make a tank “self cleaning”
include the stocking density, activity level and size of fish, the
distribution of fish around the tank, the water velocity and the
slope on the base of the tank towards it’s outlet. High stocking
densities, high fish activity levels, large fish size and high water
velocities all serve to increase the turbulence of the water and
maintain any solids particles in the water column rather than
letting them settle. This is particulalry important in raceway and D
ended tank design, where the wastes must be transported to the end
of the tank before they can be removed. Round tanks have a natural
vortexing effect on solids particles, drawing them to the centre
screen. This makes the round tank more “self cleaning”,
especially where there are small fish involved. It is for this
reason that they are in common use in hatcheries. Some tanks are
square with rounded corners. The “self cleaning” capabilities of
these is dependant on the radius of the corners; in general the
greater the radius of the corner, the more “self cleaning” the
tank is. Such tanks have the advantage of being more space efficient
than round tanks. The slope on the bottom of tanks has an important
bearing on the self cleaning ability of a tank, especially when
water velocities and stocking densities are low. A slope of between
1:20 and 1:40 towards the centre screen of a round tank or the end
of a raceway can assist in the removal of faeces and uneaten feed
and mortalities.

Water mixing (and water quality)

Tanks with well mixed waters (ie a more even water
quality across the whole tank) are advantageous in that they give
more consistent growing conditions (e.g. consistent oxygen
concentrations across the tank) and feed distribution. Round tanks
are generally well mixed systems, whereas raceways tend to be poorer
mixed systems with good water quality at the start of the raceway,
which gradually deteriorates along the length of the raceway. If the
whole holding volume of the raceway is to be used, then some fish
will inevitably occupy the lower ends of the raceway, where water
quality may be poor. D ended tanks enable greater water velocities
than would otherwise be allowed by a raceway, as only some of the
water (often between 25 - 30%) is removed at the screen and the rest
is allowed to continue flowing round the tank. This improves the
mixing in the tank and provides a good compromise between raceways
and round tanks.

Water requirements

The inflow rate to round tanks and D ended tanks is
relevant mainly to the biomass of fish held in the tank (as the
water velocities can be increased by means such as adjusting the
angle that the water enters the tank and jetting the water in under
pressure). Raceways however require a certain minimum flow to remain
self cleaning regardless of the stock held. Therefore in a systems
where the numbers of fish held in the tanks will vary, round tanks
and D ended tanks are a more efficient option where incoming water
is a limited resource.

The water velocity required for self cleaning of
faeces and uneaten feed is between 2 and 4 cm/sec.